Atmospheric Pressure Ion (API) Sources configured with Electrospray (ES) ionization interfaced to mass analyzers include at least one Electrospray sample introduction probe. Commercially available ES probes can be roughly categorized into two types, flow-through and non flow-through configurations. The non flow-through ES probes are usually configured as pre-loaded microtips where no additional sample solution is added during the spraying process. Flow-through ES probes allow the delivery of a continuous solution flow to the ES probe tip from a fluid delivery system located outside the ES chamber. ES flow-through tips have been constructed with one or more straight tube layers to simultaneously deliver liquid and gas from the attached transfer lines to the ES probe tip during operation. Flow-through ES probes are typically configured with flexible solution and gas transfer lines connected to a probe body. The liquid and gas transfer lines may be attached to the ES probes at various angles, but the single or layered tubes within ES probes have been configured as straight tubes from the point of delivery line attachment to the ES probe tip. Even in ES probes configured with a single tube for liquid sample delivery, the single tube within the ES probe body is straight after the liquid transfer line attachment point to the ES probe body. When a single layer ES probe configuration is used, the sample bearing liquid is Electrosprayed directly from the exit tip of the probe tube. When it is desirable to operate Electrospray with pneumatic nebulization assist, a second layer tube is positioned surrounding and concentric to the innermost solution introduction tube, through which nebulization gas is delivered to the ES probe tip. Three concentric tube layers have been configured in ES probes to deliver a second liquid flow layered over the sample solution with a third layer for introduction of nebulizing gas at the ES probe tip.
Electrospray probes with straight single or layered tube configurations have been positioned on or off axis in Electrospray ion sources. Electrospray probes have been mounted with the probe tip axis aligned with the ES source axis as defined by the axis of the orifice into vacuum. ES probe assemblies have been configured in a fixed on-axis position or with the ability to have the probe tip position rotated and translated in the x, y and z direction around the ES source centerline. Off-axis ES probe assemblies have also been configured where the probe straight tube axis is generally positioned to direct the Electrosprayed solution toward the ES source centerline near the centerline of the orifice into vacuum. Off axis ES probes which incorporate pneumatic nebulization assist have also been used for higher liquid flow rate applications, as is described in U.S. Pat. No. 5,495,108. An off-axis Electrospray probe configured with pneumatic nebulization assist is generally mounted at an angle ranging from .phi.=40.degree. to .phi.=90.degree. relative to the ES source vacuum orifice centerline. U.S. Pat. No. 5,495,108 even describes that an ES probe with pneumatic nebulization assist can be mounted in a position .phi.=180.degree. relative to the direction of gas flow through the vacuum orifice leading to the mass spectrometer. Analytica of Branford, Inc. has also configured ES sources with single or multiple ES probes mounted in a single source (see, Analytica's PCT patent application entitled Multiple Sample Introduction Mass Spectrometry and filed Sep. 11, 1998). In all cases, each ES probe assembly individually was configured with a straight and concentric single or layered tube assembly after the transfer line attachment points.
The straight ES probe assembly configuration requires that the entire ES probe body be angled and positioned to achieve the optimal ES probe tip position in an ES source chamber. This configuration of straight tube ES probes imposes constraints on the ES source chamber design, particularly for "off-axis" ES probe tip orientation. When off-axis ES probe mounting is used, the ES source chamber must be configured large enough to fit the ES probe body and transfer line attachments within the ES source chamber. Alternatively, the ES probe length must be increased or the ES chamber size reduced if it is desirable to position the off-axis ES probe body outside the ES source chamber with the probe assembly extending through the side wall of the ES chamber. When ES source configurations require applying kilovolt potentials to ES probes during operation, appropriate electrical insulation must be applied to any ES probes extending through the ES chamber walls. In some ES source configurations, ES probes are operated at ground potential, and kilovolt potentials are applied to surrounding electrodes. ES probes which extend through these electrodes can pass close to these electrodes and must be appropriately insulated. The surrounding electrode shapes and ES probes must be configured to accommodate "on-axis" and "off-axis" ES probe position placement while producing the desired electric fields during operation, even over a wide range of liquid flow rates.
An ES source can accommodate a sample liquid flow rate range of over 10,000 to 1. Depending on the analytical application, sample liquid can be sprayed at flow rates ranging from less than 25 nanoliters per minute to over 2.5 milliliters per minute. To achieve optimal performance over this range of liquid flow rates, ES sources can be configured to accommodate a number of ES probe configurations and a range of ES probe positions. For lower liquid flow rate applications, ES probes are generally positioned on or near the ES source centerline. With higher flow rate applications, ES probes may be positioned off the ES source centerline angled toward the centerline to optimize ES performance. To achieved added flexibility in operation, more than one ES probe can be mounted in the ES source simultaneously and even operated simultaneously. The size, complexity and cost of an ES source increases when it must accommodate the mounting of one or more ES probes in multiple positions when the ES probes are configured with straight single or multiple liquid and gas tubes after the transfer line attachment point. Particularly in low liquid flow rate applications where it is important to minimize dead volume, the liquid transfer lines are typically mounted "in-line" with the ES probe liquid sample delivery tube. The "in-line" connection of the sample delivery tube with the ES probe tube assembly may increase the ES probe length placing additional size and position constraints on the ES source and probe design.